The present invention is directed toward an inexpensive and compact apparatus for providing a loud siren or whooping sound using a minimum of space and power, avoiding use of a bulky transformer, and employing a piezoelectric transducer, a frequency-swept signal generator, and an amplifier circuit.
A variety of products from automobiles to household appliances rely upon effective alarms to notify the user of a wide variety of conditions. Many of these devices employ piezoelectric transducers that generate sounds or tones that are continuous or pulsing. Alternative sounds, such as a siren sound or a xe2x80x9cwhoopingxe2x80x9d sound are desirable because they may be more audibly distinctive. However, currently available alarms that make such sounds are large and expensive, because they use power transistors to power a transformer which then drives a piezoelectric element. Thus, a simple, inexpensive and compact alarm that does not use a transformer is desired.
The sound output of the invention is amplified by using a logic circuit consisting of a buffered voltage doubling circuit. A logic circuit is a electrical circuit that performs symbolic logic or Boolean algebraic operations. Piezoelectric transducers are sound-producing electronic devices that are preferred by industry because they are by and large extremely inexpensive, reliable, durable, and versatile. This transducer has the unique property that it undergoes a reversible mechanical deformation on the application of an electrical potential across it. Conversely, it also generates an electrical potential upon mechanical deformation. These characteristics make it highly desirable for sound-producing applications. When an oscillating potential is placed across the transducer, it vibrates at roughly the same frequency as the oscillations. These vibrations are transmitted to the ambient medium, such as air, to become sound waves. Piezoelectric transducers can also be coupled to a simple circuit in what is known as a feedback mode, well known in the art, in which there is an additional feedback terminal located on the element. In this mode, the crystal will oscillate at a natural, resonant frequency without the need for an external source for applying continuous driving oscillations. As long as the oscillations are in the range of audible sound, i.e., 20 to 20,000 Hertz, such oscillations can produce an audible signal for use as an alarm or an indicator.
Any periodic oscillation can be characterized by at least one amplitude and frequency Ordinarily, the amplitude of oscillations of interest in a piezoelectric transducer application will be dictated by the voltage swing applied across the element. By the principles explained above, it is evident that there will be a greater mechanical deformation in the crystal with greater applied voltage. The effect is roughly linear within limits, the limits being based in general on crystal composition and geometry. Thus, in the linear region, doubling the voltage swing doubles the mechanical deformation. Doubling the mechanical deformation significantly increases the amplitude of vibrations transmitted into the ambient medium. Increased amplitude of vibrations in the medium causes an increased sound level, a relationship determinable by well known physical equations.
More specifically, when a piezoelectric element possesses two terminals and a driving oscillation is placed across one while the other is clamped to a common potential such as ground, the voltage swing will be at most the amplitude of the oscillations. Thus, if an oscillation of amplitude of 5 volts is placed across one terminal, while the other is maintained at 0 volts, the maximum voltage swing will be 5 volts. This effectively caps the achievable decibel level of any sound to a value corresponding to the supply voltage. One could double the supply voltage to achieve double the voltage swing, but this has the disadvantage of added cost, and further is impractical when a piezoelectric audio circuit is to be placed in a unit having a standardized voltage supply such as an automobile. Alternatively, one could use a second supply disposed to provide the same oscillations but in a reversed polarity to double the effective voltage swing. But this approach possesses at least the same disadvantages.
The present invention therefore employs the buffered voltage doubling circuitry shown in U.S. Pat. No. 5,990,784, xe2x80x9cSchmitt Trigger Loud Alarm with Feedback.xe2x80x9d That patent is owned by the assignee of this application, and its contents are adopted by reference here.
As is shown in the ""784 patent, when a piezoelectric element possesses two terminals and a driving oscillation is placed across one, and the identical driving oscillation is placed across the other but shifted 180 degrees out of phase, the voltage swing will be about two times the amplitude of the oscillations. By xe2x80x9c180 degrees out of phasexe2x80x9d it is meant that each terminal generates a signal having a substantially square wave form, wherein one wave form is high and the other is low at any given time. Thus, if an oscillation having an amplitude of 5 volts is placed across one terminal while the other experiences the same oscillation but separated by 180 degrees of phase (half the period of the cycle), then the maximum voltage swing will be 10 volts. Higher sound pressures and louder tones result with a voltage swing of 10 volts than with a voltage swing of 5 volts.
The phase shift needed to effectively double the voltage swing across the transducer can be accomplished by use of one or more Schmitt triggers. It is believed that Schmitt triggers are particularly useful to the present invention because of their fast switching time and because they require minimal addition of components. Schmitt triggers are a special type of bistable amplifier circuit known in the art which can sustain two different voltages, each being equal in amplitude but 180 degrees out of phase. Schmitt triggers further have regenerative capability through the use of a feedback loop. In other words, a Schmitt trigger can be started or triggered by an initial pulse of only a short duration and can be maintained indefinitely (for all practical purposes) in one of its bistable states through its own feedback, without the need for an external source to supply continuing driving oscillations. Furthermore, Schmitt triggers have the added benefit of producing either a high or low output in response to a trigger signal, depending upon the state that the circuit is already in. In other words, where the input voltage is between the low and high threshold voltages of each of the stable states of a Schmitt trigger, the output of the Schmitt trigger is inverted from high to low, or vice versa. This feature can be used to place alternating voltage drops of equal magnitude across opposing terminals of a transducer, thus increasing the mechanical deformation in the transducer.
Particularly in alarm applications, what is needed is a loud sound that does not depend on the added circuit complexity of a doubled supply voltage or an additional reversed polarity supply. Loud sounds require relatively high voltages to produce relatively large amplitude vibrations in the transducer. In a special analog circuit, this might not be an obstacle. However, in a circuit containing elements that are safely and reliably operable only in a limited range of potentials, accommodations must be made to insure that those elements do not receive an electrical potential that is too high. Thus, in particular when a loud alarm sound is needed, care must be taken to separate the potentials driving the transducer from the potentials driving the more sensitive circuit elements. For example, integrated circuits often have specifications limiting the recommended power supply to 5 volts DC. If one desires to power a transducer using a substantially higher supply voltage, care must be taken to regulate the power supplied to the integrated circuit.
The voltage doubling circuit described here is used in conjunction with a frequency-swept signal generator, such as an integrated circuit like the ZSD100 made by Zetex Inc., 87 Modular Avenue, Commack, N.Y., 11725. A frequency-swept signal generator is a device that generates a signal that has an initial target frequency and a final target frequency. The frequency of the generated signal varies between the initial and final target frequencies over a target period of time. The initial frequency, final frequency, and target time are determined by inputs into the device. The signal generator can be comprised of discrete components, or can be an integrated circuit. An example of the use of discrete components is shown in U.S. Pat. No. 5,675,311. That patent is owned by the assignee of this application, and its contents are adopted by reference here.
Instead of using the signal generator with a power transistor and a transformer, the voltage doubling circuit is used. The signal generator produces an output which varies in frequency. That output is fed into the voltage doubling circuit. The voltage doubling circuit buffers the signal generating circuit from the piezoelectric transducer, and doubles the voltage driving the transducer, thus increasing the audible output of the transducer. By varying the value of external components connected to the signal generating circuit, a variety of sounds can be produced.
Accordingly, an object of the present invention is inexpensively to enable loud sounds of a siren-like or whooping character to be generated by an audio circuit that is compact and inexpensive.